The assembly and rearrangement of macromolecules within the pre- and postsynaptic compartments is critical for synapse morphogenesis, function, and plasticity, and it has been intensively investigated by neurobiologists searching for clues about fundamental mechanisms underlying neuronal function in health and disease. Compared to what we know about the structure and composition of the pre- and postsynaptic specializations, much less is known about the mechanisms that regulate the structural and functional plasticity of the synapse during development and later on in learning and memory. The goal of this project is to elucidate the role of the evolutionarily conserved PAR-1 kinase in regulating synaptic structure and function, using Drosophila as a model organism. In our preliminary studies, we have found that PAR-1, which has been implicated in polarity establishment in multiple cell types, is enriched at the neuromuscular junction (NMJ). PAR-1 gain of function and loss of function have profound and largely opposite effects on synaptic structure and function at the NMJ. We find that PAR-1 regulates Dlg, a member of the PSD-95 family of scaffolding proteins that are key regulators of synaptic organization. PAR-1 directly phosphorylates Dlg in vitro and in vivo and this phosphorylation event regulates the extrasynaptic trafficking and synaptic targeting of Dlg. These data lead to our central hypothesis that PAR-1 phosphorylation of Dlg plays a critical role in regulating synaptic differentiation and plasticity. In this application, we propose to apply an integrated approach combining biochemical, cell biological, genetic, and electrophysiological analyses to address the following questions: 1) At the molecular and cellular level, how does PAR-1-mediated phosphorylation affect Dlg function? 2) Does PAR- 1 cooperate with other kinases to regulate Dlg phosphorylation and function? 3) How is PAR-1's synaptic activity regulated during synaptogenesis? Accomplishing these goals will lay an important foundation for future forward genetic approaches aimed at identifying new genes that act in the PAR-1 pathway of synaptogenesis and signaling mechanisms that regulate this pathway under physiological conditions. Given that deregulation of PAR-1 signaling has been implicated in Alzheimer's disease and other dimentias, information to be obtained from this study will help understand the molecular basis of synaptic dysfunction in these neurodegenerative diseases and possibly other neurological disorders where prominent synaptic abnormality is featured. PUBLIC HEALTH RELEVANCE: The goal of this proposal is to achieve a deeper understanding of how neurons establish and maintain sites of communication (synapses) and what causes this process to go awry in disease conditions. Accomplishment of the proposed aims will ultimately help understand and treat a number of neurological conditions in which dysfunction of the synapses is prominently featured, such as certain neurodevelopmental disorders, learning and memory deficits, and Alzheimer's disease.